Planar cell polarity signaling in Drosophila requires the receptor Frizzled and the cytoplasmic proteins Dishevelled and Prickle. From initial, symmetric subcellular distributions in pupal wing cells, Frizzled and Dishevelled become highly enriched at the distal portion of the cell cortex. A Prickle-dependent intercellular feedback loop is described that generates asymmetric Frizzled and Dishevelled localization. In the absence of Prickle, Frizzled and Dishevelled remain symmetrically distributed. Prickle localizes to the proximal side of pupal wing cells and binds the Dishevelled DEP domain, inhibiting Dishevelled membrane localization and antagonizing Frizzled accumulation. This activity is linked to Frizzled activity on the adjacent cell surface. Prickle therefore functions in a feedback loop that amplifies differences between Frizzled levels on adjacent cell surfaces (Tree, 2002).

Using a glutathione-S-transferase (GST) pull-down assay, it was found that full-length, in vitro translated Pk^Sple (one of the three alternatively spliced forms of Prickle) binds to a GST-Dsh fusion protein. To determine the domain dependency of this interaction, domains of Pk were tested for binding to individual Dsh domains. A Pk construct containing just the conserved PET and LIM domains that are common to all Pk isoforms (PkPETLIM), but not either domain individually, is sufficient for binding to full-length Dsh. Furthermore, of the three defined Dsh domains, DIX, PDZ, and DEP, it was found that PkPETLIM binds a construct containing the DEP domain but not other Dsh domains. The binding between Pk and Dsh was confirmed, and the domain specificity was verified, using a yeast two-hybrid assay. PkPETLIM fused to a DNA binding domain interacts specifically with the Dsh DEP domain but not with the other domains of Dsh. It is concluded that Pk, via its conserved PET and LIM domains, binds the Dsh DEP domain (Tree, 2002).

Since the Dsh DEP domain is required for membrane localization, it was hypothesized that Pk antagonizes Fz activity by interfering with Dsh membrane localization. To test this, it was necessary to find conditions in which Pk activity is uncoupled from Fz/Dsh activity in the adjacent cell. Fz-dependent Dsh localization was therefore reconstituted in cultured cells. In U20S cells, transfected GFP-tagged Dsh (Dsh::GFP) can be seen in punctate patches in the cytoplasm. On cotransfection with Fz, Dsh::GFP is translocated to the cell membrane, as was seen in a similar frog animal cap assay. To test whether the physical interaction between Pk and Dsh affects Dsh membrane localization, Fz, Dsh::GFP, and Pk were cotransfected. Dsh is localized in the cytoplasm in 90% of the cotransfected cells, resembling cells transfected with Dsh::GFP alone. In contrast, Pk lacking the PET domain is severely impaired in its ability to block Dsh membrane localization. Pk, therefore, interferes with Dsh membrane association in this heterologous system and by extension may interfere with Dsh membrane localization at the proximal boundary of pupal wing cells. Since Dsh is required to generate asymmetric localization of Fz, it is suggested that blocking Dsh membrane localization also inhibits the accumulation of Fz at the proximal boundary (Tree, 2002).

Fz activity on the distal side of the cell is linked to the accumulation of Pk on the proximal side of the adjacent cell and that accumulation of Pk suppresses Fz/Dsh localization at the proximal cell cortex. In this way, Fz and Pk appear to function in a feedback loop rather than a linear signaling pathway. The ordering of these members of the PCP pathway was analyzed through epistasis analysis. The loss-of-function phenotypes of pk^pk-sple, dsh, and fz are very similar, suggesting that they activate the same signaling mechanism. However, these similarities make classical genetic epistasis tests difficult to interpret. In previous work, Fz overexpression phenotypes have been used to show that Fz signaling requires Dsh. These results were interpreted as implying that the PCP genes form a linear pathway with Dsh acting downstream of Fz. Consistent with this is the finding that Dsh localization requires Fz protein. In contrast, other observations do not fit this model. For example, localization of Fz requires Dsh, a presumed 'downstream' element of the pathway. Fz localization also requires the other presumed downstream elements Pk, the 7 cadherin Flamingo (Fmi), and novel TM protein Van gogh/Strabismis (Vang/Stbm) (Tree, 2002).

To clarify these results, several further epistasis tests were performed. Dsh was found to be required to generate a Fz overexpression phenotype and Fz is required to produce a Dsh overexpression phenotype. In addition, Pk is required for the Dsh and Fz overexpression phenotypes. These results argue against a simple linear pathway but are consistent with Fz, Dsh, and Pk participating in a feedback loop (Tree, 2002).

Evidence is provided that Fmi acts with Pk on the proximal side of the cell. Like Pk, overexpression of fmi using ptc-Gal4 causes hairs to point toward the midline of the wing. fmi overexpression clones show distal-domineering nonautonomy, phenocopying fz loss-of-function and pk overexpression clones. Thus, it is possible that Fmi could be acting similarly to Pk in feedback amplification of PCP signaling. Fmi is likely to be present at both sides of the cell, but it may act with Pk at the proximal side to suppress Fz signaling. However, Fmi must have an additional function since, within and surrounding fmi clones, very little Pk and Dsh localize to cell boundaries. In addition to functioning with Pk to suppress Fz/Dsh activity, it is suggested that Fmi could have a role in stabilizing the Fz complex at the membrane, without which no signaling occurs (Tree, 2002).

Whether differences in Pk activity between adjacent cells affect the localization of Dsh and Fz was examined. pk was overexpressed in the posterior of the wing (using UAS-pk, engrailed-Gal4), and Dsh and Fz localization were assessed near the edge of the pk overexpression domain. Dsh and Fz were relocalized to the antero-posterior cell boundaries, indicating that accumulation of Dsh and Fz occurs at interfaces between cells expressing high and low levels of Pk. Furthermore, within the posterior domain, Dsh and Fz are seen to accumulate to higher levels than in the anterior, consistent with a role for Pk in promoting PCP signaling. Two conclusions are drawn from these observations: (1) Dsh and Fz localization occur preferentially at boundaries between cells where one cell expresses high levels and the other expresses lower levels of Pk; (2) providing high levels of Pk amplifies Fz signaling, as measured by the increased accumulation of both Fz and Dsh at cell peripheries. In the wild-type, therefore, Pk at the proximal side of the cell drives Fz and Dsh accumulation at the distal side of the adjacent cell. Conversely, providing high levels of Fz induces higher levels of Pk accumulation (Tree, 2002).

Since Pk accumulates at boundaries between cells expressing different levels of Fz, and Dsh accumulates at boundaries between cells expressing different levels of Pk, it is suggested that Fz and Dsh on one side and Pk on the opposite side of an intercellular boundary form a self-organizing complex. Indeed, evidence is found for this in the posterior, pk-overexpressing domain of the same wings. In the posterior, Dsh and Fz accumulate in discrete patches around the cell peripheries. Simultaneous staining for Pk, Dsh, and Fmi, or Fz and Fmi, reveals that all four proteins colocalize to these patches. Thus, Pk, Dsh, Fmi, and Fz form self-organizing complexes bridging adjacent cells (Tree, 2002).

Various models have been proposed for the distribution of PCP information within Drosophila epithelia. Regardless of which model is correct, it is likely that this cue induces only a slight asymmetry within each cell. In the Drosophila wing, this initial small asymmetry may result in slightly higher levels of Fz accumulation or activation on the distal than on the proximal side of cells. However, this initial level of asymmetry is insufficient to polarize the cytoskeletal architecture of the cell and is undetectable as assessed by Dsh localization. Fz and Dsh localization subsequently become highly polarized through the action of a feedback amplification loop. Although the mechanism for such a feedback loop has not been demonstrated, it has previously been speculated that Fz activity on distal cell surfaces might promote expression of an inhibitory Wnt molecule that would suppress Fz activity on the adjacent cell surface. This study demonstrates suppression of Fz activity on adjacent cell surfaces but finds that it is mediated through asymmetric localization of Pk, which antagonizes Fz/Dsh accumulation by blocking cortical Dsh localization (Tree, 2002).

A model is proposed for a Pk-dependent feedback mechanism that functions across each P-D cell interface to amplify the difference between the initial levels of Fz/Dsh activity. Pk and Dsh are initially distributed nearly symmetrically, but in largely nonoverlapping distributions around the cells' apices. As a result of an initial, slight asymmetry in one or more components, Pk suppresses Fz accumulation on the proximal side of the cell by antagonizing Dsh cortical localization. Dsh association with the cortex is required for asymmetric Fz accumulation. Reduced proximal Fz accumulation then decreases Pk activity on the adjacent, distal side of the neighboring cell, allowing even greater Dsh cortical localization and Fz accumulation. Conversely, Pk localization is induced by Fz localization on the neighboring cell. The result is the highly asymmetric distribution of Fz, Dsh, and Pk observed at 30 hr APF. This mechanism is bidirectional, constituting a negative feedback loop that is predicted to be unstable; once any asymmetry is initiated, high levels of Fz/Dsh are promoted on one side of the boundary and low levels on the other. Fmi is proposed to play two roles. The localization of all components to the apical cell cortex depends on Fmi, which is thought to be on both sides of the boundary. Since Fmi overexpression mimics Pk overexpression, it is proposed that Fmi is activated selectively on the proximal side of the cell, where it works with Pk to suppress Fz/Dsh activity. In essence, the function of this regulatory loop depends on the balance of forces regulating Dsh membrane association on either side of the boundary. Fz recruits Dsh to the cell cortex, and Pk blocks this recruitment. Once an initial asymmetry is induced, these forces become different on either side of the boundary, and the feedback mechanism amplifies the differences (Tree, 2002).

Because fmi overexpression produces polarity phenotypes very similar to those seen with pk overexpression, it is likely that Fmi activity is also polarized across cell boundaries and participates in blocking Fz/Dsh accumulation on the distal side of the interface. However, Fmi has been proposed to exist on both sides of the boundary, and Fmi is required for the efficient localization of Fz, Dsh, and Pk to boundaries. Fmi, therefore, appears to have an additional, complex-stabilizing function. Diego (Dgo), an ankyrin repeat protein, is required for PCP signaling and localizes to P-D cell boundaries, though it has not been possible to resolve on which side of the boundary Diego is found. Dgo may function as a scaffold for assembly of PCP-signaling components (Tree, 2002).

The core PCP protein Van Gogh (Vang, also known as Strabismus [Stbm], a transmembrane protein, is likely to be involved in the feedback amplification mechanism. In a vang/stbm mutant background, Fz is localized symmetrically to the membrane in the same manner as in a pk null background, and vang/stbm has been proposed to function downstream of pk. Interestingly, in Xenopus and zebrafish, Stbm binds Dsh and seems to toggle its activity between the canonical Wnt and a PCP-like signaling pathway. Stbm antagonizes canonical Wnt signaling and activates a PCP-like pathway that regulates convergent extension and induces JNK signaling. It is proposed that both fly and vertebrate Vang/Stbm may function together with Pk, facilitating PCP signaling by antagonizing Dsh activity (Tree, 2002).

The feedback amplification mechanism described here provides a clue to understanding the long-standing problem of domineering nonautonomy. Loss-of-function clones of fz, vang/stbm, and to a lesser extent pk induce polarity phenotypes in neighboring wild-type tissue. The feedback loop model predicts that loss of Fz will disrupt the localization of these components in the neighboring distal cells, causing their polarity to reverse. Furthermore, if the immediate clone neighbors have reversed polarity, this could then cause the reversal to propagate over a distance. This phenomenon is observed near the lateral borders of a fz clone, where Pk accumulation is seen to run parallel to the clone, even at a distance from the mutant cells. Thus, the reversal in Pk localization may underlie, in part, the domineering nonauontomy observed within wild-type tissue distal to fz clones (Tree, 2002).

Once asymmetrically localized, the Fz/Dsh complex must direct reorganization of the cytoskeleton by both determining the location for prehair initiation and by limiting the number of prehairs to one. Rho and Rho-associated kinase directly regulate myosins to limit the number of prehairs. The recently described Formin homology protein, Daam1, links Dsh to Rho during vertebrate PCP-like signaling, and a Drosophila homolog may function similarly. However, further studies will be required to understand how localized Fz and Dsh orient prehair initiation (Tree, 2002).

Planar polarity decisions in the wing of Drosophila involve the assembly of asymmetric protein complexes containing the conserved receptor Frizzled. This study analyses the role of the Van Gogh/strabismus gene in the formation of these complexes and in determination of cell polarization. The Strabismus protein becomes asymmetrically localized to the proximal edge of cells. In the absence of strabismus activity, the planar polarity proteins Dishevelled and Prickle are mislocalized in the cell. Strabismus binds directly to Dishevelled and Prickle and is able to recruit them to membranes. Furthermore, the putative PDZ-binding motif at the C terminus of Strabismus is not required for its function. A two-step model is proposed for assembly of Frizzled-containing asymmetric protein complexes at cell boundaries. First, Strabismus acts together with Frizzled and the atypical cadherin Flamingo to mediate apicolateral recruitment of planar polarity proteins including Dishevelled and Prickle. In the second phase, Dishevelled and Prickle are required for these proteins to become asymmetrically distributed on the proximodistal axis (Bastock, 2003).

The subcellular localiaation of Stbm protein was investigated during wing morphogenesis using both a Stbm-YFP (Stbm yellow fluorescent protein) expressing transgene and using specific antibodies raised against Stbm. During the third instar stage, Stbm-YFP in the wing pouch localizes unevenly around apicolateral cell boundaries. Based on its molecular homology as a multi-pass transmembrane protein, it is assumed that Stbm is present in the outer cell membrane. At 18 hours of pupal life, a similar pattern is seen, Stbm-YFP still being distributed patchily in an apicolateral ring. By 24 hours, there is preferential distribution of Stbm-YFP to proximodistal cell boundaries; this distribution is clearly present at 28 hours and persists until at least 32 hours, which corresponds to the time of trichome initiation. The pattern seen with Stbm antibodies confirms that Stbm-YFP is a faithful reporter of Stbm protein distribution (Bastock, 2003).

The timecourse and distribution of Stbm broadly fits that described for other planar polarity proteins such as Fmi, Fz, Dsh and Pk-Sple. Consistent with this, good colocalization is found between Stbm-YFP and other polarity proteins. The localization of Stbm-YFP to the adherens junction zone was confirmed by costaining for Armadillo distribution. Conversely, Stbm-YFP shows no overlap with the distribution of Discs-Large, which is localized in the septate junction region. Mosaic analysis revealed that Stbm-YFP becomes preferentially distributed to the proximal edges of cells with no appreciable accumulation at distal edges (Bastock, 2003).

The three putative multipass transmembrane proteins Fmi, Fz and Stbm all play important roles in the first step of localizing planar polarity proteins to the apicolateral adherens junction zone. It is thought that Fmi acts at the top of the hierarchy in this process, since, in its absence, negligible amounts of any planar polarity proteins become apicolaterally localized. Stbm is also key, because, in its absence, both Fz and Fmi recruitment are reduced. Additionally, Stbm is also required for Dsh apicolateral recruitment and for efficient localization of Pk to membranes. Fz is not significantly required for apicolateral recruitment of Fmi, but is partly needed for apicolateral localization of Stbm and is absolutely required for apicolateral localization of Dsh. Hence, in the absence of Fmi, Fz or Stbm, one or more planar polarity proteins do not become apicolaterally localized and the process of asymmetric localization on the proximodistal axis does not occur (Bastock, 2003).

An important question is which of these factors are directly binding together, in the process of apicolateral recruitment. So far no direct protein interactions have been reported for Fmi, although it is tempting to speculate that Fmi might bind directly to Fz and Stbm in the process of apicolateral recruitment. However, Fz is able to recruit Dsh to membranes in a heterologous cell type, suggesting that these factors directly interact. In addition, vertebrate Stbm and Dsh homologs have been shown to directly interact. Direct interactions are shown between Drosophila Stbm and Dsh, and Stbm and Pk. This suggests a model in which Dsh and Pk both become apicolaterally localized as a result of direct interactions with Fz and Stbm. Notably, in the absence of Stbm, Pk accumulates in the cytoplasm, suggesting that its interaction with Stbm is important for regulating its level in the cell in addition to its subcellular localization (Bastock, 2003).

At the stage when the planar polarity proteins are apicolaterally localized, but prior to the stage when they are asymmetrically localized on the proximodistal axis of the wing, it is possible that they are present in either 'symmetric' or 'asymmetric' complexes assembled across cell-cell boundaries. If the complexes were symmetric, then Fmi, Fz, Stbm, Pk and Dsh would all be present in a complex together on the same side of the cell-cell boundary. Such symmetric complexes would then subsequently evolve into asymmetric complexes, with Fz/Dsh at distal cell edges and Stbm/Pk at proximal cell edges and Fmi on both sides. Alternatively, the initial apicolateral complexes formed could be asymmetric, with Fz/Dsh always on the opposite side of the cell-cell boundary from Stbm/Pk. These asymmetric complexes would initially be randomly oriented relative to the axes of the wing, but would gradually become aligned to the proximodistal axis. The possibility is favored that planar polarity protein complexes are initially symmetric, since Stbm directly interacts with Dsh and these molecules colocalize during earlier stages of wing development. However, it has been reported that Pk and Dsh-GFP do not precisely colocalize in early pupal wings: this observation supports the early presence of asymmetric complexes (Bastock, 2003).

After the apicolateral recruitment of planar polarity proteins, over a number of hours their localization alters such that they become asymmetrically distributed on the proximodistal axis of the wing. Although Dsh and Pk play negligible roles in the apicolateral recruitment of proteins, both are required for this subsequent proximodistal redistribution. Since overexpression of both factors leads to the accumulation of polarity proteins at apicolateral cell boundaries, it is suggested that they both function to promote the assembly and/or stabilization of protein complexes. Removal of the function of the planar polarity gene dgo also blocks asymmetric proximodistal localization but not apicolateral localization of other polarity proteins. Furthermore, overexpression of Dgo causes an accumulation of other polarity proteins at cell boundaries similar to that seen when Dsh and Pk are overexpressed. Therefore, it is proposed that Dsh and Pk act together with Dgo in the assembly of asymmetric complexes (Bastock, 2003).

Recently, it has been proposed that the function of Pk in asymmetric complex assembly is to antagonize Dsh localization to membranes. This model is mechanistically attractive, in providing an explanation for the formation of asymmetric complexes in which Dsh and Pk are found on opposite sides of cell-cell boundaries. However, it is found that in the presence of Stbm, Dsh and Pk will colocalize at the same membranes. Furthermore, it was not possible to show an effect of overexpressing Pk on the association of Fz and Dsh at membranes. In addition, high level Pk expression in vivo does not cause Dsh to lose its membrane localization but instead appears to increase levels of Dsh at the membrane. Resolution of these issues will require a more detailed understanding of the composition and properties of the protein complexes involved (Bastock, 2003).

Frizzled (Fz) signaling regulates the establishment of planar cell polarity (PCP). The PCP genes prickle and strabismus are thought to antagonize Fz signaling. They act in the same cell, R4, adjacent to that in which the Fz/PCP pathway is required in the Drosophila eye. Stbm and Pk interact physically; Stbm recruits Pk to the cell membrane. Through this interaction, Pk affects Stbm membrane localization and can cause clustering of Stbm. Pk is also known to interact with Dsh and is thought to antagonize Dsh by affecting its membrane localization. Thus the data suggest that the Stbm/Pk complex modulates Fz/Dsh activity, resulting in a symmetry-breaking step during polarity signaling (Jenny, 2003).

pk function is required in the R4 precursor, as opposed to fz PCP signaling in R3, for control of polarity establishment. Stbm, a transmembrane protein also required in R4, interacts genetically and physically with Pk. This interaction is important for the recruitment of Pk to the plasma membrane. In Xenopus animal-cap explants, Stbm and Pk relocalize each other to subdomains of the membrane. A model is proposed of how Pk/Stbm might regulate Fz/Dsh signaling activity (Jenny, 2003).

The in vitro molecular interaction between Pk and Stbm and their mutual relocalization in Xenopus animal caps suggest that they form multiprotein complexes. Several pieces of evidence indicate that the physical interaction is physiologically important: (1) correct membrane localization of Pk depends on stbm function because in stbm mutant tissue Pk staining is diffuse and absent (or strongly reduced) at the membrane; (2) Pk and Stbm interact genetically by mutually enhancing each other's GOF and LOF phenotypes in the eye; (3) pk is necessary for PCP signaling in the R4 precursor, the same cell in which stbm is required; (4) expression of the interacting domains of Pk or Stbm interferes with polarity establishment. In particular, a subfragment of 131 amino acids of the C-terminus of Pk, required for the molecular interaction, is sufficient to affect polarity (Jenny, 2003).

Both Pk and Stbm act as if they antagonize Fz signaling. (1) In zebrafish, Stbm overexpression can prevent Wnt11 from rescuing a wnt11 mutation. (2) In the Drosophila wing, overexpression of Pk leads to wing hairs pointing towards the source of the overexpressed protein, behaving like a fz LOF clone (whereas overexpression of Fz leads to hairs pointing away from the Fz source). stbm LOF clones show the opposite behavior to fz LOF clones: wing hairs point away from the mutant patch, consistent with the mutant tissue having a higher Fz-activity (Jenny, 2003).

In the Drosophila eye, evidence that pk acts antagonistically to fz comes from the fact that the Notch-signaling-responsive R4-specific reporter mdelta0.5-lacZ is expressed for a prolonged period in both R3/R4 precursors in a pk^sple1 mutant. This is explained if Fz activity in the R4 precursor is increased, resulting in higher levels of Dl there. This in turn leads to N activation and concomitant mdelta0.5-lacZ reporter expression in both cells of the R3/R4 pair. Conversely, in fz and dsh mutant eye discs (where Fz signaling is absent or reduced and thus Dl should not be upregulated) N-signaling activity and mdelta-lacZ expression is initially reduced in both cells. Fz activity is also antagonized by stbm in the eye. Mosaic analysis of stbm shows that it has the capability to instruct a cell to become R4 as long as the other cell of the R3/R4 pair is mutant for stbm. Therefore, in such an all-or-nothing situation, Stbm in the R3 precursor can override a positive signal of Fz, resulting in a cell fate switch to R4 fate (Jenny, 2003).

In a wild-type situation with all PCP components present in both cells, it is crucial that Stbm activity is higher in R4 than in R3 to ensure proper Fz-signaling regulation. Therefore it is an intriguing possibility that a Pk/Stbm complex in the R4 precursor ensures such higher Stbm activity, and the associated higher Fz repression there is important for a proper R3/R4 cell fate decision (Jenny, 2003).

How does the Stbm/Pk complex regulate Fz-signaling activity? During PCP establishment in the wing, Fz, Dsh, Dgo, Fmi and Pk are initially localized uniformly around the apical circumference of wing cells. During and after PCP signaling, these proteins relocalize and become differentially enriched: Pk concentrates on the proximal side of the cell, whereas Fz and Dsh become enriched distally. Fmi becomes enriched at both sides (Jenny, 2003).

In the eye, the situation is analogous. During PCP establishment, signaling components at the R3/R4 cell border are relocalized from a uniform to a more restricted pattern. Stbm-YFP is localized on the R4 but not on the R3 side, and Fz-GFP ends up on the R3 but not the R4 side. The analogy between the R4/R3 and proximal/distal cell borders is corroborated by the genetic requirements in R3 and R4: the distally localized factors Dsh and Fz are required in R3, while proximally localized Pk is required in R4. Fmi is localized on both poles of each wing cell and also required in both cells of the R3 and R4 pair. The function of fmi has been linked to both proposed complexes, the 'Fz/Dsh side' (fmi is required for apical localization of both Fz and Dsh) and the Stbm/Pk complex. In addition to the genetic interactions between fmi and stbm or pk, a reduced membrane staining of Pk in fmi^- clones in wing cells has been shown (Jenny, 2003).

How do these changes in localization occur? Localization studies in Drosophila and Xenopus suggest that Pk and Stbm influence each other's localization and form clusters in subdomains of the cell membrane. Interestingly, such Stbm/Pk complexes also affect Fz-dependent Dsh membrane localization. Thus it is an intriguing possibility that the patches observed in animal cap cells upon coinjection of Stbm with Pk represent the result of a similar, though unpolarized, symmetry-breaking step during PCP signaling (Jenny, 2003).

The PET/LIM domain of Pk can interact with the DEP-domain/C-terminus of Dsh. This interaction has been suggested to prevent Dsh membrane recruitment. Also, the C-terminus of Stbm can interact with Dsh as long as the PDZ domain is present. Since the data suggest that Pk regulates the activity and localization of Stbm, this regulation might promote or stabilize the interactions of Dsh with Stbm and/or Pk, thereby helping to pull Dsh away from a Fz-signaling complex. The Stbm/Pk complexes could then cause active release of Dsh from the membrane or target it for degradation, resulting in low levels of Dsh (and by inference Fz) at places where Pk and Stbm are enriched. Furthermore, in the R3 cell (or distally in the wing) an unknown factor might act to prevent either the formation of the Stbm/Pk complex or its effect on Dsh (Jenny, 2003).

In conclusion, Pk and Stbm form a functional complex during PCP signaling in Drosophila and during convergent extension in Xenopus. Interestingly, in zebrafish, in addition to its function in convergent extension, Stbm is also required for the caudal migration of hindbrain motor neurons. This function of Stbm is independent of Dsh and the PCP genes tested so far. It will be interesting to determine whether Stbm and Pk function together in this context as well (Jenny, 2003).

Epithelial planar cell polarity (PCP) is evident in the cellular organization of many tissues in vertebrates and invertebrates. In mammals, PCP signalling governs convergent extension during gastrulation and the organization of a wide variety of structures, including the orientation of body hair and sensory hair cells of the inner ear. In Drosophila melanogaster, PCP is manifest in adult tissues, including ommatidial arrangement in the compound eye and hair orientation in wing cells. PCP establishment requires the conserved Frizzled/Dishevelled PCP pathway. Mutations in PCP-pathway-associated genes cause aberrant orientation of body hair or inner-ear sensory cells in mice, or misorientation of ommatidia and wing hair in Drosophila. This study provides mechanistic insight into Frizzled/Dishevelled signalling regulation. The ankyrin-repeat protein Diego binds directly to Dishevelled and promotes Frizzled signalling. Dishevelled can also be bound by the Frizzled PCP antagonist Prickle. Strikingly, Diego and Prickle compete with one another for Dishevelled binding, thereby modulating Frizzled/Dishevelled activity and ensuring tight control over Frizzled PCP signalling (Jenny, 2005).